[XCOFF][AIX] Generate symbol table entries with llvm-readobj

Summary:

This patch implements main entry and auxiliary entries of symbol table generation for llvm-readobj on AIX.
The source code of aix_xcoff_xlc_test8.o (compile with xlc) is:

-bash-4.2$ cat test8.c
extern int i;
extern int TestforXcoff;
extern int fun(int i);
static int static_i;
char* p="abcd";
int fun1(int j) {
  static_i++;
  j++;
  j=j+*p;
  return j;
}
int main() {
  i++;
  fun(i);
  return fun1(i);
}

Patch provided by DiggerLin

Differential Revision: https://reviews.llvm.org/D65240

llvm-svn: 370097